In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added products (such as pipeline-quality substitute natural gas, hydrogen, methanol, higher hydrocarbons, ammonia and electrical power) from lower-fuel-value carbonaceous feedstocks (such as petroleum coke, resids, asphaltenes, coal and biomass) is receiving renewed attention.
Such lower-fuel-value carbonaceous feedstocks can be gasified at elevated temperatures and pressures to produce a synthesis gas stream that can subsequently be converted to such value-added products.
One advantageous gasification process is hydromethanation, in which the carbonaceous feedstock is converted in a fluidized-bed hydromethanation reactor in the presence of a catalyst source and steam at moderately-elevated temperatures and pressures to directly produce a methane-enriched synthesis gas stream raw product. This is distinct from conventional gasification processes, such as those based on partial combustion/oxidation of a carbon source at highly-elevated temperatures and pressures (thermal gasification, typically non-catalytic), where a syngas (carbon monoxide+hydrogen) is the primary product (little or no methane is directly produced), which can then be further processed to produce methane (via catalytic methanation, see reaction (III) below) or any number of other higher hydrocarbon products.
Hydromethanation processes and the conversion/utilization of the resulting methane-rich synthesis gas stream to produce value-added products are disclosed, for example, in U.S. Pat. Nos. 3,828,474, 3,958,957, 3,998,607, 4,057,512, 4,092,125, 4,094,650, 4,204,843, 4,243,639, 4,468,231, 4,500,323, 4,541,841, 4,551,155, 4,558,027, 4,604,105, 4,617,027, 4,609,456, 5,017,282, 5,055,181, 6,187,465, 6,790,430, 6,894,183, 6,955,695, US2003/0167691A1, US2006/0265953A1, US2007/0000177A1, US2007/0083072A1, US2007/0277437A1, US2009/0048476A1, US2009/0090056A1, US2009/0090055A1, US2009/0165383A1, US2009/0166588A1, US2009/0165379A1, US2009/0170968A1, US2009/0165380A1, US2009/0165381A1, US2009/0165361A1, US2009/0165382A1, US2009/0169449A1, US2009/0169448A1, US2009/0165376A1, US2009/0165384A1, US2009/0217582A1, US2009/0220406A1, US2009/0217590A1, US2009/0217586A1, US2009/0217588A1, US2009/0218424A1, US2009/0217589A1, US2009/0217575A1, US2009/0229182A1, US2009/0217587A1, US2009/0246120A1, US2009/0259080A1, US2009/0260287A1, US2009/0324458A1, US2009/0324459A1, US2009/0324460A1, US2009/0324461A1, US2009/0324462A1, US2010/0071235A1, US2010/0071262A1, US2010/0120926A1, US2010/0121125A1, US2010/0168494A1, US2010/0168495A1, US2010/0179232A1, US2010/0287835A1, US2010/0287836A1, US2010/0292350A1, US2011/0031439A1, US2011/0062012A1, US2011/0062721A1, US2011/0062722A1, US2011/0064648A1, US2011/0088896A1, US2011/0088897A1, US2011/0146978A1, US2011/0146979A1, US2011/0207002A1, US2011/0217602A1, US2011/0262323A1, US2012/0046510A1, US2012/0060417A1, US2012/0102836A1, US2012/0102837A1, US2012/0213680A1, US2012/0271072A1, US2012/0305848A1, US2013/0046124A1, US2013/0042824A1, US2013/0172640A1, US2014/0094636A1, WO2011/029278A1, WO2011/029282A1, WO2011/029283A1, WO2011/029284A1, WO2011/029285A1, WO2011/063608A1 and GB1599932, all of which are hereby incorporated by reference. See also Chiaramonte et al, “Upgrade Coke by Gasification”, Hydrocarbon Processing, September 1982, pp. 255-257; and Kalina et al, “Exxon Catalytic Coal Gasification Process Predevelopment Program, Final Report”, Exxon Research and Engineering Co., Baytown, TX, FE236924, December 1978, all of which are also hereby incorporated by reference.
The hydromethanation of a carbon source typically involves four theoretically separate reactions:Steam carbon: C+H2O→CO+H2  (I)Water-gas shift: CO+H2O→H2+CO2  (II)CO Methanation: CO+3H2→CH4+H2O  (III)Hydro-gasification: 2H2+C→CH4  (IV)
In the hydromethanation reaction, the first three reactions (I-III) predominate to result in the following overall net reaction:2C+2H2O→CH4+CO2  (V)
The overall hydromethanation reaction is essentially thermally balanced; however, due to process heat losses and other energy requirements (such as required for evaporation of moisture entering the reactor with the feedstock), some heat must be added to maintain the thermal balance.
Referring to FIG. 1, in one variation of the hydromethanation process, required carbon monoxide, hydrogen and heat energy can also at least in part be generated in situ by feeding oxygen into the hydromethanation reactor. See, for example, previously incorporated US2010/0287835A1 and US2011/0062721A1, as well as commonly-owned US2012/0046510A1, US2012/0060417A1, US2012/0102836A1, US2012/0102837A1, US2013/0046124A1, US2013/0042824A1, US2013/0172640A1 and US2014/0094636A1, and US2010/0076235A1, which is hereby incorporated by reference. The following exothermic reactions will provide the heat energy to balance the heat losses from the reactor and the energy requirement of the steam carbon and other endothermic reactions:C+½O2→CO  (VI)CO+½O2→CO2  (VII)C+O2→CO2  (VIII)H2+½O2→H2O  (IX)
The result is a “direct” methane-enriched raw product gas stream also containing substantial amounts of hydrogen, carbon monoxide and carbon dioxide which can, for example, be directly utilized as a medium BTU energy source, or can be processed to result in a variety of higher-value product streams such as pipeline-quality substitute natural gas, high-purity hydrogen, methanol, ammonia, higher hydrocarbons, carbon dioxide (for enhanced oil recovery and industrial uses) and electrical energy.
A char by-product stream is also produced in addition to the methane-enriched raw product gas stream. The solid char by-product contains unreacted carbon, entrained hydromethanation catalyst and other inorganic components of the carbonaceous feedstock. The by-product char may contain 20 wt % or more carbon depending on the feedstock composition and hydromethanation conditions.
This by-product char is periodically or continuously removed from the hydromethanation reactor, and typically sent to a catalyst recovery and recycle operation to improve economics and commercial viability of the overall process. The nature of catalyst components associated with the char extracted from a hydromethanation reactor and methods for their recovery are disclosed, for example, in previously incorporated US2007/0277437A1, US2009/0165383A1, US2009/0165382A1, US2009/0169449A1 and US2009/0169448A1, as well as commonly-owned US2011/0262323A1 and US2012/0213680A1. Catalyst recycle can be supplemented with makeup catalyst as needed, such as disclosed in previously incorporated US2009/0165384A1.
Catalyst recovery, and particularly catalyst recovery from high-ash char products, is often difficult to perform economically due to several factors, including the volume of by-product char material produced.
In addition, the residue of the char after catalyst recovery still contains a significant amount of carbon that is unconverted and is effectively removed from the process.
It would, therefore, be desirable to find a way to more completely and efficiently utilize the carbon in the char by-product from the hydromethanation reactor while reducing the volume of material that is ultimately sent for catalyst recovery.
And further, in some cases it may be desirable to produce additional electric power and steam from the char by-product.